Breakable coupling system for a turbojet fan shaft

ABSTRACT

The invention relates to a rotary shaft arrangement carrying equipment at one end and extending rearwards from the equipment, the shaft ( 1 ) being supported by a first bearing ( 2 ) behind the equipment, and by a second bearing ( 3 ) behind the first bearing ( 2 ), the first bearing ( 2 ) being carried by a casing ( 4 ) surrounding the shaft ( 1 ) and extending rearwards from the first bearing ( 2 ) to a stator structure ( 5 ) to which the casing ( 4 ) is fastened by screws ( 6 ) that extend parallel to the shaft ( 1 ) and that are fusible in traction, the arrangement being characterized by the fact that the second bearing ( 3 ) is disposed with radial clearance (J) in a bore ( 7 ) of an annular support ( 8 ) secured to the stator structure ( 5 ), and is fastened to said annular support ( 8 ) by screws ( 9 ) that are parallel to the shaft ( 1 ) and that are fusible in shear, whereby the second bearing ( 3 ) can bear against said support ( 8 ) in the event of said screws ( 9 ) rupturing. The invention relates more particularly to the support for the fan shaft of a turbojet.

The invention relates to a shaft support that breaks on the appearanceof unbalance.

More precisely, the invention relates to a rotary shaft arrangementcarrying equipment at one end and extending rearwards from theequipment, the shaft being supported by a first bearing behind theequipment, and by a second bearing behind the first bearing, the firstbearing being carried by a casing surrounding the shaft and extendingrearwards from the first bearing to a stator structure to which thecasing is fastened by screws that extend parallel to the shaft and thatare fusible in traction.

Such an arrangement is described in FR 2 752 024. By way of example, theequipment is a turbojet fan driven and supported by the shaft of alow-pressure turbine.

If a blade of the fan breaks, a large unbalance occurs on thefan-support shaft, thereby generating cyclical loads and vibration thatthe first bearing of the shaft support communicates to the stationaryportions of the machine, with a high risk of causing damage. Rupture ofthe screws that are fusible in traction leads to the casing becominguncoupled some small number of shaft revolutions after a large unbalanceappears, e.g. due to a blade of the fan breaking.

Once the casing has become uncoupled, the first bearing no longersupports the fan, and the fan shaft is supported only by the secondbearing which in turn begins to transmit cyclical loads and vibration tothe stator structure via its support.

FR 2 752 024 provides for an embodiment in which the annular support ofthe second bearing is connected to the support casing of the firstbearing. In the event of the screws that are fusible in tractionrupturing, the front end of the low-pressure shaft line becomesfloating, and can oscillate quite strongly in the radial direction, andmight rub against the high-pressure shaft line that is coaxial therewithand rotates at a significantly higher speed, which can lead to damage inboth shaft lines.

FR 2 752 024 also makes provision for limiting the travel of thelow-pressure shaft line after rupture of the screws that are fusible intraction by mounting a rib on the stator around the casing so as tocontain its movement, or another rib fixed to the stator and terminatedby shoes situated behind the second bearing and surrounding thelow-pressure shaft line at a small distance therefrom. When the travelof the low-pressure shaft is limited by the shoes of the other rim,situated at a small distance from the shaft, friction naturally occursbetween the low-pressure shaft and the shoes, which can damage thelow-pressure shaft.

The object of the invention is to propose a shaft arrangement as definedin the introduction, which mitigates the above-mentioned drawbacks.

The invention achieves this object by the fact that the second bearingis disposed with radial clearance in a bore of an annular supportsecured to the stator structure, and is fastened to said annular supportby screws that are parallel to the shaft and that are fusible in shear,whereby the second bearing can bear against said support in the event ofsaid screws rupturing.

Most advantageously, the radial clearance is calibrated so as to avoidfriction between the shaft and other elements, whether stationary ormoving.

Preferably, the second bearing includes an outer ring which presents anoutwardly-directed radial annular flange that is pressed against a sideface of the annular support by the fusible screws.

According to a second advantageous characteristic of the invention, theouter ring is maintained centered in the bore of the annular support bya plurality of studs extending parallel to the shaft, said studs beingcalibrated to shear in the event of the second bearing becominguncoupled.

Preferably, the studs are provided on the annular support and extendfrom the periphery of the flange.

According to another advantageous characteristic of the invention, meansare provided to prevent the outer ring from turning in the event of thesecond bearing becoming uncoupled.

Advantageously, the means for preventing the outer bearing from turningin the event of uncoupling comprise at least one pin anchored in theannular support passing with clearance through an orifice formed in theflange.

Most advantageously, that pin includes a pin head bearing against theoutside face of the flange so as to prevent the second bearing frommoving axially in the event of the second bearing becoming uncoupled.

Thus, in the event of uncoupling, the outer ring comes to bear againstthe periphery of the bore, and is prevented from turning by the pinswhich also prevent the outer ring of the second bearing from movingaxially.

Other advantages and characteristics of the invention will appear onreading the following description given by way of non-limiting exampleand with reference to the accompanying drawings, in which:

FIG. 1 is a general view of the invention;

FIG. 2 shows, on a larger scale, how the rear bearing of the shaft ismounted;

FIG. 3 is a front view of the outer ring in a preferred embodiment ofthe invention; and

FIG. 4 shows, likewise on a larger scale, how the rear bearing of theshaft is mounted, this figure being in section on line IV-IV of FIG. 3.

FIG. 1 shows the front end of a shaft 1 of a low-pressure turbine of abypass turbojet of axis X, which shaft is supported by a front bearing 2and a rear bearing 3. The shaft 1 supports, cantilevered out in front ofthe front bearing 2, a fan of axis X (not shown in the drawings), and itis supported at its rear end by a third bearing that is not shown in thedrawings.

The front bearing 2 is carried by a casing 4 which surrounds the shaft 1and which extends rearwards from the front bearing 2 to a statorstructure 5, to which it is united by a plurality of screws 6 disposedparallel to the axis X, and which are fusible in traction. Each of thesescrews 6 has a middle portion of small section which ruptures when thefront bearings 2 is subjected to cyclical loads in the event of anunbalance appearing, due to a blade of the fan rupturing or becomingsignificantly deformed.

The rear bearing 3 is disposed in a bore 7 of axis X in a rigid support8, which is fixed to the stator structure 5, or which is integrated inthe stator structure 5, and it is fastened to said support 8 by means ofscrews 9 of axis X that are suitable for breaking in shear when the rearbearing 3 is subjected to large cyclical loads.

As shown in FIG. 2, the bore 7 of the support 8 is of a diameter greaterthan the outside diameter of the outer peripheral element 10 of the rearbearing 3, this outer peripheral element 10 presenting anoutwardly-directed radial annular flange 11 pressed against a side faceof the support 8 by the screws 9.

There thus exists radial clearance J between the outer peripheral wall10 and the radially-inner wall 12 of the support 8 that defines the bore7.

In the event of the screws 9 rupturing in shear the radial travel of therear bearing 3 is defined by the radial clearance J, the outerperipheral wall 10 of the rear bearing 3 then coming to bear against theradially-inner wall 12 of the support 8.

When the fan creates a large amount of unbalance, the front bearing 2 issubjected to cyclical loads, which are compensated by axial forces onthe screws 6 that are fusible in traction, which break one after anotherafter a very small number of revolutions of the shaft 1. The frontbearing 2 no longer carries the fan, and the rear bearing 3 is subjectedin turn to cyclical loads that can cause the screws 9 that are fusiblein shear to rupture. However, the rear bearing 3 is maintained pressingradially against the radially-inner wall 12 of the support 8.

The clearance J is calibrated in such a manner that the shaft 1 of thelow-pressure turbine does not rub against any stationary or movingelements of the turbojet, and in particular does not rub against thehigh-pressure shaft which connects the high-pressure turbine to thehigh-pressure compressor, which shaft is coaxial around the low-pressureshaft 1, and turns at a significantly greater speed.

The clearance J may be limited to 4 millimeters (mm), for example, andis preferably limited to 3 mm, so as to reduce very greatly any risk ofcontact between the shafts when the turbojet is operating at high speed,and to reduce forces when it is operating at low speed.

Thus, as shown in FIG. 3, the radial annular flange 11 of the peripheralelement 10, which in fact constitutes the outer ring of the secondbearing 3, has a plurality of bores 20, e.g. 12 bores, for passingfusible screws 9. The flange 11 also has three through orifices 21 whichare preferably circular and which are regularly distributed around theaxis X of the shaft. These orifices 21 are for receiving with suitableclearance pins 22 that are anchored in the annular support 8, as shownin FIG. 4, so as to prevent the outer ring 10 from turning in the eventof the second bearing 3 becoming uncoupled, while still allowing theouter ring 10 to bear against the radially-inner wall 12 of the support8. Each pin 22 has a pin head 23 at its free end which is pressedlightly against the outer front face of the annular flange 11 so as toprevent the second bearing 3 from moving axially while still leaving acertain amount of radial travel available for the second bearing 3.

Reference 25 designates a stud secured to the annular support 8 andprojecting from the periphery of the flange 11. The annular support 8preferably has three studs 25 regularly distributed around the peripheryof the flange, which studs serve to center the outer ring 10 in the bore7 during assembly. These studs 25 are calibrated so as to be shearedafter the fusible screws 9 have sheared, as soon as unbalance appears.

1. A rotary shaft arrangement carrying equipment at one end andextending rearwards from the equipment, the shaft (1) being supported bya first bearing (2) behind the equipment, and by a second bearing (3)behind the first bearing (2), the first bearing (2) being carried by acasing (4) surrounding the shaft (1) and extending rearwards from thefirst bearing (2) to a stator structure (5) to which the casing (4) isfastened by screws (6) that extend parallel to the shaft (1) and thatare fusible in traction, the arrangement being characterized by the factthat the second bearing (3) is disposed with radial clearance (J) in abore (7) of an annular support (8) secured to the stator structure (5),and is fastened to said annular support (8) by screws (9) that areparallel to the shaft (1) and that are fusible in shear, whereby thesecond bearing (3) can bear against said support (8) in the event ofsaid screws (9) rupturing.
 2. An arrangement according to claim 1,characterized by the fact that the radial clearance (J) is calibrated soas to avoid friction between the shaft (1) and other elements, whetherstationary or moving.
 3. An arrangement according to claim 2,characterized by the fact that the radial clearance (J) is no more than3 mm.
 4. An arrangement according to any one of claims 1 to 3,characterized by the fact that the equipment is an entry fan of aturbojet.
 5. An arrangement according to any one of claims 1 to 4,characterized by the fact that the second bearing (3) includes an outerring (10) which presents an outwardly-directed radial annular flange(11) that is pressed against a side face of the annular support (8) bythe fusible screws (9).
 6. An arrangement according to claim 5,characterized by the fact that the outer ring (10) is maintainedcentered in the bore (7) of the annular support (8) by a plurality ofstuds (25) extending parallel to the shaft (1), said studs beingcalibrated to shear in the event of the second bearing becominguncoupled.
 7. An arrangement according to claim 6, characterized by thefact that the studs are provided on the annular support (8) and extendfrom the periphery of the flange (11).
 8. An arrangement according toclaim 7, characterized by the fact that the annular support (8) hasthree studs that are regularly distributed around the axis of the bore.9. An arrangement according to any one of claims 1 to 8, characterizedby the fact that means are provided to prevent the outer ring (10) fromturning in the event of the second bearing (3) becoming uncoupled. 10.An arrangement according to claim 9, characterized by the fact that themeans for preventing the outer bearing (10) from turning in the event ofuncoupling comprise at least one pin (22) anchored in the annularsupport (8) passing with clearance through an orifice (21) formed in theflange (11).
 11. An arrangement according to claim 10, characterized bythe fact that pin (22) includes a pin head (23) bearing against theoutside face of the flange (11) so as to prevent the second bearing (3)from moving axially in the event of the second bearing becominguncoupled.
 12. An arrangement according to claim 10 or claim 11,characterized by the fact that it has three pins (22) that are regularlydistributed around the axis of the bore (7).